/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/*
 *
 *
 *
 *
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.util.*;
import java.util.concurrent.atomic.AtomicInteger;
import java.security.AccessControlContext;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.security.PrivilegedExceptionAction;
import java.security.PrivilegedActionException;
import java.security.AccessControlException;
import sun.security.util.SecurityConstants;

/**
 * Factory and utility methods for {@link Executor}, {@link
 * ExecutorService}, {@link ScheduledExecutorService}, {@link
 * ThreadFactory}, and {@link Callable} classes defined in this
 * package. This class supports the following kinds of methods:
 *
 * <ul>
 * <li> Methods that create and return an {@link ExecutorService}
 * set up with commonly useful configuration settings.
 * <li> Methods that create and return a {@link ScheduledExecutorService}
 * set up with commonly useful configuration settings.
 * <li> Methods that create and return a "wrapped" ExecutorService, that
 * disables reconfiguration by making implementation-specific methods
 * inaccessible.
 * <li> Methods that create and return a {@link ThreadFactory}
 * that sets newly created threads to a known state.
 * <li> Methods that create and return a {@link Callable}
 * out of other closure-like forms, so they can be used
 * in execution methods requiring {@code Callable}.
 * </ul>
 *
 * @author Doug Lea
 * @since 1.5
 */
public class Executors {

  /**
   * Creates a thread pool that reuses a fixed number of threads
   * operating off a shared unbounded queue.  At any point, at most
   * {@code nThreads} threads will be active processing tasks.
   * If additional tasks are submitted when all threads are active,
   * they will wait in the queue until a thread is available.
   * If any thread terminates due to a failure during execution
   * prior to shutdown, a new one will take its place if needed to
   * execute subsequent tasks.  The threads in the pool will exist
   * until it is explicitly {@link ExecutorService#shutdown shutdown}.
   *
   * @param nThreads the number of threads in the pool
   * @return the newly created thread pool
   * @throws IllegalArgumentException if {@code nThreads <= 0}
   */
  public static ExecutorService newFixedThreadPool(int nThreads) {
    return new ThreadPoolExecutor(nThreads, nThreads,
        0L, TimeUnit.MILLISECONDS,
        new LinkedBlockingQueue<Runnable>());
  }

  /**
   * Creates a thread pool that maintains enough threads to support
   * the given parallelism level, and may use multiple queues to
   * reduce contention. The parallelism level corresponds to the
   * maximum number of threads actively engaged in, or available to
   * engage in, task processing. The actual number of threads may
   * grow and shrink dynamically. A work-stealing pool makes no
   * guarantees about the order in which submitted tasks are
   * executed.
   *
   * @param parallelism the targeted parallelism level
   * @return the newly created thread pool
   * @throws IllegalArgumentException if {@code parallelism <= 0}
   * @since 1.8
   */
  public static ExecutorService newWorkStealingPool(int parallelism) {
    return new ForkJoinPool
        (parallelism,
            ForkJoinPool.defaultForkJoinWorkerThreadFactory,
            null, true);
  }

  /**
   * Creates a work-stealing thread pool using all
   * {@link Runtime#availableProcessors available processors}
   * as its target parallelism level.
   *
   * @return the newly created thread pool
   * @see #newWorkStealingPool(int)
   * @since 1.8
   */
  public static ExecutorService newWorkStealingPool() {
    return new ForkJoinPool
        (Runtime.getRuntime().availableProcessors(),
            ForkJoinPool.defaultForkJoinWorkerThreadFactory,
            null, true);
  }

  /**
   * Creates a thread pool that reuses a fixed number of threads
   * operating off a shared unbounded queue, using the provided
   * ThreadFactory to create new threads when needed.  At any point,
   * at most {@code nThreads} threads will be active processing
   * tasks.  If additional tasks are submitted when all threads are
   * active, they will wait in the queue until a thread is
   * available.  If any thread terminates due to a failure during
   * execution prior to shutdown, a new one will take its place if
   * needed to execute subsequent tasks.  The threads in the pool will
   * exist until it is explicitly {@link ExecutorService#shutdown
   * shutdown}.
   *
   * @param nThreads the number of threads in the pool
   * @param threadFactory the factory to use when creating new threads
   * @return the newly created thread pool
   * @throws NullPointerException if threadFactory is null
   * @throws IllegalArgumentException if {@code nThreads <= 0}
   */
  public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {
    return new ThreadPoolExecutor(nThreads, nThreads,
        0L, TimeUnit.MILLISECONDS,
        new LinkedBlockingQueue<Runnable>(),
        threadFactory);
  }

  /**
   * Creates an Executor that uses a single worker thread operating
   * off an unbounded queue. (Note however that if this single
   * thread terminates due to a failure during execution prior to
   * shutdown, a new one will take its place if needed to execute
   * subsequent tasks.)  Tasks are guaranteed to execute
   * sequentially, and no more than one task will be active at any
   * given time. Unlike the otherwise equivalent
   * {@code newFixedThreadPool(1)} the returned executor is
   * guaranteed not to be reconfigurable to use additional threads.
   *
   * @return the newly created single-threaded Executor
   */
  public static ExecutorService newSingleThreadExecutor() {
    return new FinalizableDelegatedExecutorService
        (new ThreadPoolExecutor(1, 1,
            0L, TimeUnit.MILLISECONDS,
            new LinkedBlockingQueue<Runnable>()));
  }

  /**
   * Creates an Executor that uses a single worker thread operating
   * off an unbounded queue, and uses the provided ThreadFactory to
   * create a new thread when needed. Unlike the otherwise
   * equivalent {@code newFixedThreadPool(1, threadFactory)} the
   * returned executor is guaranteed not to be reconfigurable to use
   * additional threads.
   *
   * @param threadFactory the factory to use when creating new threads
   * @return the newly created single-threaded Executor
   * @throws NullPointerException if threadFactory is null
   */
  public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory) {
    return new FinalizableDelegatedExecutorService
        (new ThreadPoolExecutor(1, 1,
            0L, TimeUnit.MILLISECONDS,
            new LinkedBlockingQueue<Runnable>(),
            threadFactory));
  }

  /**
   * Creates a thread pool that creates new threads as needed, but
   * will reuse previously constructed threads when they are
   * available.  These pools will typically improve the performance
   * of programs that execute many short-lived asynchronous tasks.
   * Calls to {@code execute} will reuse previously constructed
   * threads if available. If no existing thread is available, a new
   * thread will be created and added to the pool. Threads that have
   * not been used for sixty seconds are terminated and removed from
   * the cache. Thus, a pool that remains idle for long enough will
   * not consume any resources. Note that pools with similar
   * properties but different details (for example, timeout parameters)
   * may be created using {@link ThreadPoolExecutor} constructors.
   *
   * @return the newly created thread pool
   */
  public static ExecutorService newCachedThreadPool() {
    return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
        60L, TimeUnit.SECONDS,
        new SynchronousQueue<Runnable>());
  }

  /**
   * Creates a thread pool that creates new threads as needed, but
   * will reuse previously constructed threads when they are
   * available, and uses the provided
   * ThreadFactory to create new threads when needed.
   *
   * @param threadFactory the factory to use when creating new threads
   * @return the newly created thread pool
   * @throws NullPointerException if threadFactory is null
   */
  public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {
    return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
        60L, TimeUnit.SECONDS,
        new SynchronousQueue<Runnable>(),
        threadFactory);
  }

  /**
   * Creates a single-threaded executor that can schedule commands
   * to run after a given delay, or to execute periodically.
   * (Note however that if this single
   * thread terminates due to a failure during execution prior to
   * shutdown, a new one will take its place if needed to execute
   * subsequent tasks.)  Tasks are guaranteed to execute
   * sequentially, and no more than one task will be active at any
   * given time. Unlike the otherwise equivalent
   * {@code newScheduledThreadPool(1)} the returned executor is
   * guaranteed not to be reconfigurable to use additional threads.
   *
   * @return the newly created scheduled executor
   */
  public static ScheduledExecutorService newSingleThreadScheduledExecutor() {
    return new DelegatedScheduledExecutorService
        (new ScheduledThreadPoolExecutor(1));
  }

  /**
   * Creates a single-threaded executor that can schedule commands
   * to run after a given delay, or to execute periodically.  (Note
   * however that if this single thread terminates due to a failure
   * during execution prior to shutdown, a new one will take its
   * place if needed to execute subsequent tasks.)  Tasks are
   * guaranteed to execute sequentially, and no more than one task
   * will be active at any given time. Unlike the otherwise
   * equivalent {@code newScheduledThreadPool(1, threadFactory)}
   * the returned executor is guaranteed not to be reconfigurable to
   * use additional threads.
   *
   * @param threadFactory the factory to use when creating new threads
   * @return a newly created scheduled executor
   * @throws NullPointerException if threadFactory is null
   */
  public static ScheduledExecutorService newSingleThreadScheduledExecutor(
      ThreadFactory threadFactory) {
    return new DelegatedScheduledExecutorService
        (new ScheduledThreadPoolExecutor(1, threadFactory));
  }

  /**
   * Creates a thread pool that can schedule commands to run after a
   * given delay, or to execute periodically.
   *
   * @param corePoolSize the number of threads to keep in the pool, even if they are idle
   * @return a newly created scheduled thread pool
   * @throws IllegalArgumentException if {@code corePoolSize < 0}
   */
  public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
    return new ScheduledThreadPoolExecutor(corePoolSize);
  }

  /**
   * Creates a thread pool that can schedule commands to run after a
   * given delay, or to execute periodically.
   *
   * @param corePoolSize the number of threads to keep in the pool, even if they are idle
   * @param threadFactory the factory to use when the executor creates a new thread
   * @return a newly created scheduled thread pool
   * @throws IllegalArgumentException if {@code corePoolSize < 0}
   * @throws NullPointerException if threadFactory is null
   */
  public static ScheduledExecutorService newScheduledThreadPool(
      int corePoolSize, ThreadFactory threadFactory) {
    return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory);
  }

  /**
   * Returns an object that delegates all defined {@link
   * ExecutorService} methods to the given executor, but not any
   * other methods that might otherwise be accessible using
   * casts. This provides a way to safely "freeze" configuration and
   * disallow tuning of a given concrete implementation.
   *
   * @param executor the underlying implementation
   * @return an {@code ExecutorService} instance
   * @throws NullPointerException if executor null
   */
  public static ExecutorService unconfigurableExecutorService(ExecutorService executor) {
    if (executor == null) {
      throw new NullPointerException();
    }
    return new DelegatedExecutorService(executor);
  }

  /**
   * Returns an object that delegates all defined {@link
   * ScheduledExecutorService} methods to the given executor, but
   * not any other methods that might otherwise be accessible using
   * casts. This provides a way to safely "freeze" configuration and
   * disallow tuning of a given concrete implementation.
   *
   * @param executor the underlying implementation
   * @return a {@code ScheduledExecutorService} instance
   * @throws NullPointerException if executor null
   */
  public static ScheduledExecutorService unconfigurableScheduledExecutorService(
      ScheduledExecutorService executor) {
    if (executor == null) {
      throw new NullPointerException();
    }
    return new DelegatedScheduledExecutorService(executor);
  }

  /**
   * Returns a default thread factory used to create new threads.
   * This factory creates all new threads used by an Executor in the
   * same {@link ThreadGroup}. If there is a {@link
   * java.lang.SecurityManager}, it uses the group of {@link
   * System#getSecurityManager}, else the group of the thread
   * invoking this {@code defaultThreadFactory} method. Each new
   * thread is created as a non-daemon thread with priority set to
   * the smaller of {@code Thread.NORM_PRIORITY} and the maximum
   * priority permitted in the thread group.  New threads have names
   * accessible via {@link Thread#getName} of
   * <em>pool-N-thread-M</em>, where <em>N</em> is the sequence
   * number of this factory, and <em>M</em> is the sequence number
   * of the thread created by this factory.
   *
   * @return a thread factory
   */
  public static ThreadFactory defaultThreadFactory() {
    return new DefaultThreadFactory();
  }

  /**
   * Returns a thread factory used to create new threads that
   * have the same permissions as the current thread.
   * This factory creates threads with the same settings as {@link
   * Executors#defaultThreadFactory}, additionally setting the
   * AccessControlContext and contextClassLoader of new threads to
   * be the same as the thread invoking this
   * {@code privilegedThreadFactory} method.  A new
   * {@code privilegedThreadFactory} can be created within an
   * {@link AccessController#doPrivileged AccessController.doPrivileged}
   * action setting the current thread's access control context to
   * create threads with the selected permission settings holding
   * within that action.
   *
   * <p>Note that while tasks running within such threads will have
   * the same access control and class loader settings as the
   * current thread, they need not have the same {@link
   * java.lang.ThreadLocal} or {@link
   * java.lang.InheritableThreadLocal} values. If necessary,
   * particular values of thread locals can be set or reset before
   * any task runs in {@link ThreadPoolExecutor} subclasses using
   * {@link ThreadPoolExecutor#beforeExecute(Thread, Runnable)}.
   * Also, if it is necessary to initialize worker threads to have
   * the same InheritableThreadLocal settings as some other
   * designated thread, you can create a custom ThreadFactory in
   * which that thread waits for and services requests to create
   * others that will inherit its values.
   *
   * @return a thread factory
   * @throws AccessControlException if the current access control context does not have permission
   * to both get and set context class loader
   */
  public static ThreadFactory privilegedThreadFactory() {
    return new PrivilegedThreadFactory();
  }

  /**
   * Returns a {@link Callable} object that, when
   * called, runs the given task and returns the given result.  This
   * can be useful when applying methods requiring a
   * {@code Callable} to an otherwise resultless action.
   *
   * @param task the task to run
   * @param result the result to return
   * @param <T> the type of the result
   * @return a callable object
   * @throws NullPointerException if task null
   */
  public static <T> Callable<T> callable(Runnable task, T result) {
    if (task == null) {
      throw new NullPointerException();
    }
    return new RunnableAdapter<T>(task, result);
  }

  /**
   * Returns a {@link Callable} object that, when
   * called, runs the given task and returns {@code null}.
   *
   * @param task the task to run
   * @return a callable object
   * @throws NullPointerException if task null
   */
  public static Callable<Object> callable(Runnable task) {
    if (task == null) {
      throw new NullPointerException();
    }
    return new RunnableAdapter<Object>(task, null);
  }

  /**
   * Returns a {@link Callable} object that, when
   * called, runs the given privileged action and returns its result.
   *
   * @param action the privileged action to run
   * @return a callable object
   * @throws NullPointerException if action null
   */
  public static Callable<Object> callable(final PrivilegedAction<?> action) {
    if (action == null) {
      throw new NullPointerException();
    }
    return new Callable<Object>() {
      public Object call() {
        return action.run();
      }
    };
  }

  /**
   * Returns a {@link Callable} object that, when
   * called, runs the given privileged exception action and returns
   * its result.
   *
   * @param action the privileged exception action to run
   * @return a callable object
   * @throws NullPointerException if action null
   */
  public static Callable<Object> callable(final PrivilegedExceptionAction<?> action) {
    if (action == null) {
      throw new NullPointerException();
    }
    return new Callable<Object>() {
      public Object call() throws Exception {
        return action.run();
      }
    };
  }

  /**
   * Returns a {@link Callable} object that will, when called,
   * execute the given {@code callable} under the current access
   * control context. This method should normally be invoked within
   * an {@link AccessController#doPrivileged AccessController.doPrivileged}
   * action to create callables that will, if possible, execute
   * under the selected permission settings holding within that
   * action; or if not possible, throw an associated {@link
   * AccessControlException}.
   *
   * @param callable the underlying task
   * @param <T> the type of the callable's result
   * @return a callable object
   * @throws NullPointerException if callable null
   */
  public static <T> Callable<T> privilegedCallable(Callable<T> callable) {
    if (callable == null) {
      throw new NullPointerException();
    }
    return new PrivilegedCallable<T>(callable);
  }

  /**
   * Returns a {@link Callable} object that will, when called,
   * execute the given {@code callable} under the current access
   * control context, with the current context class loader as the
   * context class loader. This method should normally be invoked
   * within an
   * {@link AccessController#doPrivileged AccessController.doPrivileged}
   * action to create callables that will, if possible, execute
   * under the selected permission settings holding within that
   * action; or if not possible, throw an associated {@link
   * AccessControlException}.
   *
   * @param callable the underlying task
   * @param <T> the type of the callable's result
   * @return a callable object
   * @throws NullPointerException if callable null
   * @throws AccessControlException if the current access control context does not have permission
   * to both set and get context class loader
   */
  public static <T> Callable<T> privilegedCallableUsingCurrentClassLoader(Callable<T> callable) {
    if (callable == null) {
      throw new NullPointerException();
    }
    return new PrivilegedCallableUsingCurrentClassLoader<T>(callable);
  }

  // Non-public classes supporting the public methods

  /**
   * A callable that runs given task and returns given result
   */
  static final class RunnableAdapter<T> implements Callable<T> {

    final Runnable task;
    final T result;

    RunnableAdapter(Runnable task, T result) {
      this.task = task;
      this.result = result;
    }

    public T call() {
      task.run();
      return result;
    }
  }

  /**
   * A callable that runs under established access control settings
   */
  static final class PrivilegedCallable<T> implements Callable<T> {

    private final Callable<T> task;
    private final AccessControlContext acc;

    PrivilegedCallable(Callable<T> task) {
      this.task = task;
      this.acc = AccessController.getContext();
    }

    public T call() throws Exception {
      try {
        return AccessController.doPrivileged(
            new PrivilegedExceptionAction<T>() {
              public T run() throws Exception {
                return task.call();
              }
            }, acc);
      } catch (PrivilegedActionException e) {
        throw e.getException();
      }
    }
  }

  /**
   * A callable that runs under established access control settings and
   * current ClassLoader
   */
  static final class PrivilegedCallableUsingCurrentClassLoader<T> implements Callable<T> {

    private final Callable<T> task;
    private final AccessControlContext acc;
    private final ClassLoader ccl;

    PrivilegedCallableUsingCurrentClassLoader(Callable<T> task) {
      SecurityManager sm = System.getSecurityManager();
      if (sm != null) {
        // Calls to getContextClassLoader from this class
        // never trigger a security check, but we check
        // whether our callers have this permission anyways.
        sm.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);

        // Whether setContextClassLoader turns out to be necessary
        // or not, we fail fast if permission is not available.
        sm.checkPermission(new RuntimePermission("setContextClassLoader"));
      }
      this.task = task;
      this.acc = AccessController.getContext();
      this.ccl = Thread.currentThread().getContextClassLoader();
    }

    public T call() throws Exception {
      try {
        return AccessController.doPrivileged(
            new PrivilegedExceptionAction<T>() {
              public T run() throws Exception {
                Thread t = Thread.currentThread();
                ClassLoader cl = t.getContextClassLoader();
                if (ccl == cl) {
                  return task.call();
                } else {
                  t.setContextClassLoader(ccl);
                  try {
                    return task.call();
                  } finally {
                    t.setContextClassLoader(cl);
                  }
                }
              }
            }, acc);
      } catch (PrivilegedActionException e) {
        throw e.getException();
      }
    }
  }

  /**
   * The default thread factory
   */
  static class DefaultThreadFactory implements ThreadFactory {

    private static final AtomicInteger poolNumber = new AtomicInteger(1);
    private final ThreadGroup group;
    private final AtomicInteger threadNumber = new AtomicInteger(1);
    private final String namePrefix;

    DefaultThreadFactory() {
      SecurityManager s = System.getSecurityManager();
      group = (s != null) ? s.getThreadGroup() :
          Thread.currentThread().getThreadGroup();
      namePrefix = "pool-" +
          poolNumber.getAndIncrement() +
          "-thread-";
    }

    public Thread newThread(Runnable r) {
      Thread t = new Thread(group, r,
          namePrefix + threadNumber.getAndIncrement(),
          0);
      if (t.isDaemon()) {
        t.setDaemon(false);
      }
      if (t.getPriority() != Thread.NORM_PRIORITY) {
        t.setPriority(Thread.NORM_PRIORITY);
      }
      return t;
    }
  }

  /**
   * Thread factory capturing access control context and class loader
   */
  static class PrivilegedThreadFactory extends DefaultThreadFactory {

    private final AccessControlContext acc;
    private final ClassLoader ccl;

    PrivilegedThreadFactory() {
      super();
      SecurityManager sm = System.getSecurityManager();
      if (sm != null) {
        // Calls to getContextClassLoader from this class
        // never trigger a security check, but we check
        // whether our callers have this permission anyways.
        sm.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);

        // Fail fast
        sm.checkPermission(new RuntimePermission("setContextClassLoader"));
      }
      this.acc = AccessController.getContext();
      this.ccl = Thread.currentThread().getContextClassLoader();
    }

    public Thread newThread(final Runnable r) {
      return super.newThread(new Runnable() {
        public void run() {
          AccessController.doPrivileged(new PrivilegedAction<Void>() {
            public Void run() {
              Thread.currentThread().setContextClassLoader(ccl);
              r.run();
              return null;
            }
          }, acc);
        }
      });
    }
  }

  /**
   * A wrapper class that exposes only the ExecutorService methods
   * of an ExecutorService implementation.
   */
  static class DelegatedExecutorService extends AbstractExecutorService {

    private final ExecutorService e;

    DelegatedExecutorService(ExecutorService executor) {
      e = executor;
    }

    public void execute(Runnable command) {
      e.execute(command);
    }

    public void shutdown() {
      e.shutdown();
    }

    public List<Runnable> shutdownNow() {
      return e.shutdownNow();
    }

    public boolean isShutdown() {
      return e.isShutdown();
    }

    public boolean isTerminated() {
      return e.isTerminated();
    }

    public boolean awaitTermination(long timeout, TimeUnit unit)
        throws InterruptedException {
      return e.awaitTermination(timeout, unit);
    }

    public Future<?> submit(Runnable task) {
      return e.submit(task);
    }

    public <T> Future<T> submit(Callable<T> task) {
      return e.submit(task);
    }

    public <T> Future<T> submit(Runnable task, T result) {
      return e.submit(task, result);
    }

    public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
        throws InterruptedException {
      return e.invokeAll(tasks);
    }

    public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
        long timeout, TimeUnit unit)
        throws InterruptedException {
      return e.invokeAll(tasks, timeout, unit);
    }

    public <T> T invokeAny(Collection<? extends Callable<T>> tasks)
        throws InterruptedException, ExecutionException {
      return e.invokeAny(tasks);
    }

    public <T> T invokeAny(Collection<? extends Callable<T>> tasks,
        long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
      return e.invokeAny(tasks, timeout, unit);
    }
  }

  static class FinalizableDelegatedExecutorService
      extends DelegatedExecutorService {

    FinalizableDelegatedExecutorService(ExecutorService executor) {
      super(executor);
    }

    protected void finalize() {
      super.shutdown();
    }
  }

  /**
   * A wrapper class that exposes only the ScheduledExecutorService
   * methods of a ScheduledExecutorService implementation.
   */
  static class DelegatedScheduledExecutorService
      extends DelegatedExecutorService
      implements ScheduledExecutorService {

    private final ScheduledExecutorService e;

    DelegatedScheduledExecutorService(ScheduledExecutorService executor) {
      super(executor);
      e = executor;
    }

    public ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit) {
      return e.schedule(command, delay, unit);
    }

    public <V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit) {
      return e.schedule(callable, delay, unit);
    }

    public ScheduledFuture<?> scheduleAtFixedRate(Runnable command, long initialDelay, long period,
        TimeUnit unit) {
      return e.scheduleAtFixedRate(command, initialDelay, period, unit);
    }

    public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command, long initialDelay,
        long delay, TimeUnit unit) {
      return e.scheduleWithFixedDelay(command, initialDelay, delay, unit);
    }
  }

  /**
   * Cannot instantiate.
   */
  private Executors() {
  }
}
